1. Field of the Invention
The present invention relates to a protective device in which a heating element is energized during a malfunction, whereby the heating element is heated and a low-melting metal element is fused.
2. Related Art of the Invention
The conventional current fuses in which low-melting metal element composed of lead, tin, antimony, or the like are fused by overcurrent are widely known as protective devices for cutting off such overcurrent. Protective devices comprising heating elements and low-melting metal elements are also known as protective devices capable of preventing not only overcurrents but also overvoltages (Japanese Patent No. 2,790,433; Japanese Patent Application Laid-Open No. 8-161990, etc.).
FIG. 9 is a circuit diagram of an overvoltage prevention device featuring such a protective device 1p. FIG. 10A and FIG. 10B are respectively a plane view and a cross sectional view of the protective device 1p. The protective device 1p is obtained by the sequential stacking of the following elements on a substrate 2: a heating element 3 (formed by applying or otherwise spreading a resistance paste), an insulating layer 4, and a low-melting metal element 5 composed of a fuse material. In the drawing, the numerals 6a and 6b are electrodes for the heating element, and the numerals 7a and 7b are electrodes for the low-melting metal element. In addition, the numeral 8 is an inside seal composed of solid flux or the like and designed to seal the low-melting metal element 5 in order to prevent the surface of this low-melting metal element 5 from being oxidized; and the numeral 9 is an outside seal composed of a material whose melting point or softening point is higher than that of the low-melting metal element 5 and designed not to allow molten material to flow outside the device during the fusion of the low-melting metal element 5.
In the overvoltage prevention device shown in FIG. 9 and obtained using the protective device 1p, the electrode terminals of, for example, a lithium ion battery or other device to be protected are connected to terminals A1 and A2; and the electrode terminals of, for example, a charger or other device connected to the device to be protected are connected to terminals B1 and B2. With this overvoltage prevention device, when the lithium ion battery is charged and a reverse voltage higher than the breakdown voltage is applied to a Zener diode D, base current ib flows in an abrupt manner, substantial collector current ic greater than the base current ib is caused to flow across the heating element 3, and the heating element 3 is heated. This heat is transmitted to the low-melting metal element 5 on the heating element 3, the low-melting metal element 5 is fused, and the application of overvoltage to the terminals A1 and A2 is prevented.
With the overvoltage prevention device in FIG. 9, however, current continues to flow through the heating element 3 even after the low-melting metal element 5 has been fused by the overvoltage. An overvoltage prevention device whose circuitry is shown in FIG. 11 is also known. FIG. 12A and FIG. 12B are respectively a plane view and a cross sectional view of the protective device 1q used in this overvoltage prevention device. In this protective device 1q, two heating elements 3 are connected by means of an intermediate electrode 6c, and a low-melting metal element 5 is disposed thereon so as to allow an insulating layer 4 to intervene therebetween.
In the overvoltage prevention device shown in FIG. 11, the heat generated by the heating elements 3 fuses the low-melting metal element 5 at two locations (5a and 5b), completely cutting off electric power to the heating elements 3 following this type of fusion.
Also known is a protective device 1r in which the arrangement in which a heating element 3 and low-melting metal element 5 are stacked so as not to allow an insulating layer 4 to intervene therebetween, is replaced by an arrangement in which a heating element 3 and a low-melting metal element 5 are arranged in a planar configuration on a substrate 2, as shown in FIG. 13. In the drawing, the numerals 6d, 6e, 6f, and 6g are electrodes, and the numeral 8 is an inside seal consisting of a flux coating film (Japanese Patent Application Laid-open Nos. 10-116549 and 10-116550).
In situations such as those encountered with the protective device 1p or 1q shown in FIGS. 10A and 10B or FIGS. 12A and 12B, stacking the heating element 3 and the low-melting metal element 5 so as to allow the insulating layer 4 to intervene therebetween makes it difficult to reduce the operating time (that is, the time from the energizing of the heating element 3 to the fusing of the low-melting metal element 5) because the heat-up of the low-melting metal element 5 is slowed down by the delay in heat transfer due to the presence of the insulating layer 4 during the heating of the heating element 3. When glass components are used for the insulating layer 4, the insulating layer 4 flows during heating, creating a risk that fusion characteristics will be adversely affected.
In a structure in which a heating element 3 and a low-melting metal element 5 are arranged in a planar configuration on a substrate 2 (as in the protective device 1r in FIG. 13), the planar configuration of the elements cannot be miniaturized because separate planar spaces are required for arranging the heating element 3 and the low-melting metal element 5. Consequently, the protective device 1r is larger than the above-described protective device 1p or 1q, which are obtained by stacking the heating element 3 and the low-melting metal element 5 so as to allow the insulating layer 4 to intervene therebetween.
Merely reducing the size of the protective device 1r in this case will result in a smaller surface area for the electrodes, making it impossible to fuse the low-melting metal element 5 because of low rated current or insufficient heat generation.
Another feature of the protective device 1r is that the heat from the heating element 3 during heating is transferred via the electrode 6g and the substrate 2, slowing down the heat-up of the low-melting metal element 5 and hence increasing the operating time. Mounting the protective device 1r on the base circuit substrate with the aid of solder in order in an attempt to enhance the thermal conductivity of the substrate 2 (and thus to eliminate the delay in the operating time) is disadvantageous because the mounting solder melts before the fusion of the low-melting metal element 5, and the protective device 1r separates from the base circuit substrate. In addition, lowering the melting point of the low-melting metal element 5 in order to eliminate the delay in the operating time has an adverse effect on the reflow resistance of the protective device 1r during mounting, makes it impossible to use automatic mounting, and turns the protective device 1r into a hand-mounted component.
An object of the present invention is to overcome the shortcomings of prior art and to make it possible to miniaturize the devices and to reduce the operating time without reducing the rated current in a protective device in which a low-melting metal element is fused by the energizing of a heating element.
The inventor perfected the present invention upon discovering that to cause fusion in a protective device in which a heating element and a low-melting metal element are formed on a substrate, and the low-melting metal element is fused by the heat generated by the heating element, it is important that adequate space be provided for the low-melting metal element to wet the surface and to spread thereover during melting, resulting in fusion; that the fusion of the low-melting metal element can be facilitated by making it easier for the molten low-melting metal element to wet the heating element, electrodes, and other components in contact with the low-melting metal element; that the section wetted by the fused low-melting metal element or the area in the vicinity of this section may in this case serve as the location in which the material is heated by this heating element; and that there is, therefore, no need to stack the low-melting metal element on the heating element so as to allow the insulating layer to intervene therebetween and to cause the entire heating element to generate heat in the same manner as in the conventional protective device 1p or 1q in FIGS. 10A and 10B or FIGS. 12A and 12B.
Specifically, the present invention provides a protective device comprising a heating element and a low-melting metal element on a substrate, the low-melting metal element being fused by heat generated by the heating element, wherein the heating element and the low-melting metal element are stacked so as not to allow an insulating layer to intervene therebetween.
Because the heating element and the low-melting metal element in the protective device of the present invention are stacked so as not to allow an insulating layer to intervene therebetween, the temperature of the low-melting metal element can increase rapidly during the heating of the heating element, and the operating time can be reduced. In addition, there is no risk that the insulating layer will have an adverse effect on the fusion characteristics of the low-melting metal element, as in the conventional protective devices.
It is also possible to miniaturize the protective device without reducing the rated current of the protective device, compared with the conventional protective devices, because of an increase in the proportion of the surface area or volume of the low-melting metal element in the protective device.
This and other objects, features and advantages of the present invention are described in or will become apparent from the following detailed description of the invention.